Heretofore, as shown in FIG. 1, in the optical fiber transmission system 1 which is transmitted via optical fiber, the transmitting data is reproduced from receiving signals received via the optical fiber.
More specifically, when an optical input S1 transmitted via the optical fiber is received at a photodiode 2 for example, which converts it into an input signal S2 to be supplied to a current/voltage converting circuit 3.
An input voltage signal S3 converted into voltage by the current/voltage convertor is supplied to an optical input receiving unit 4, in-which the voltage signal S3 is amplified by a main amplifier 5 so as to reproduce the transmission data based on the received signal S4 obtained therefrom.
Thus, in the optical fiber transmission system 1, the transmission data is reproduced from the received signal S4 outputted from the main amplifier 5, but also the received signal S4 is supplied to an alarm signal generation circuit 6 to judge whether the optical input S1 to be inputted exists or not.
In the alarm signal generation circuit 6, the received signal S4 is inputted to a peak hold circuit 7 to detect the peak level and the detection result is supplied to a comparison circuit 8. Then the comparison circuit 8 judges whether the optical input S1 is inputted or not depending on the comparison result between the peak level detection signal S5 inputted from the peak hold circuit 7 and the reference voltage, and outputs an alarm signal S6.
For example, in the case where the signal level of the received signal S4 drops when the optical fiber is disconnected from the connector, the comparison circuit 8 judges that the optical input S1 is not inputted and then the peak level is lower than the reference voltage, so as to make the logical value of the alarm signal S6 rise to an "H" level which is outputted to the peripheral device.
Since there is a possibility that noises which are generated in the preprocessing circuits (i.e., photodiode 2, current/voltage converter 3, etc.) may be mixed in the peak level detection signal S5 that is inputted to a comparison circuit 8, the hysteresis characteristic is provided in the comparison circuit 8 and the hysteresis range is adjustable by the hysteresis adjusting signal S7.
Furthermore, since the dynamic range of the peak level detection signal S5 that is inputted to the comparison circuit 8 fluctuates approximately ten times depending on the type of the photodiode 2 (e.g., avalanche photodiode and pin photodiode) and the difference in gain of the current/voltage V.sub.ref as shown in FIGS. 2A and 2B converter 3, the reference voltage can be also adjusted by changing the threshold level setting signal S8.
However, in the case of the optical fiber transmission system 1, whenever the input level of the receiving signal S4 fluctuates dye to the combination of circuit elements used in the preprocessing circuits and the variations in their operating characteristics and the operating temperature, the hysteresis range and the reference voltage must be adjusted by the hysteresis adjusting signal S7 which is inputted from the adjusting terminal and by the threshold level setting signal S8. Therefore, a great deal of labor is required for adjustment of the signals.
Furthermore, as shown in FIG. 3, a conventional hysteresis variable type comparison circuit 8 generates a voltage in proportion to the hysteresis by providing comparison outputs V.sub.OUT1 and V.sub.OUT2 to be outputted from the differential input stage constituted by a pair of NPN-type transistors Q1 and Q2 through buffer transistors Q3 and Q4 to the base of transistors Q5 and Q6 so as to feedback the offset current corresponding to the comparison outputs VOUT1 and V.sub.OUT2 to load resistances R1 and R2 (in which the resistance values are both R).
More specifically, in the comparison circuit 8, the hysteresis range (R.times.I2) can be adjusted by adjusting a current I2 being carried through the current source 9 connected to the common emitter of the transistors Q5 and Q6 which constitute the latch circuit.
However, in this type of comparison circuit 8, since the current I2 decreases at a fixed ratio and the gain of the latch circuit also decreases as the input amplitude decreases if hysteresis changes at a constant ratio to the input amplitude at, in the case where the input amplitude is small and the signal level of the receiving signal S4 is near to the reference voltage there occurs problems that the transistors Q5 and Q6 do not perform their switching function, hysteresis does not function properly and the input dynamic range comes narrow.